1. Field of the Invention
The present invention relates to a display device and an active matrix substrate used therein, and in particular, to a switching element used for the display device and an active matrix substrate used therein, and methods for producing the display device and the active matrix substrate.
2. Description of the Related Art
In recent years, significant progress has been made in the use of low-profile display (FPD) devices such as a liquid crystal display and an organic EL (electro luminescence) display device. Of the low-profile display devices, an active matrix type display device capable of high-quality display in particular has three-terminal elements such as an amorphous silicon TFT (thin-film transistor), a polysilicon TFT and two-terminal elements such as an MIM (Metal-Insulator-Metal) used as switching elements thereon.
These switching elements are formed on a substrate constituting a display device in a process of manufacturing the display device. The switching elements are generally formed by repeating a film deposition process and a patterning process a plurality of times. For instance, in the case of a TFT, it is necessary to repeat the film deposition process and a patterning process at least three times, and these processes are usually repeated four to eight times. In the case of the MIM, it is necessary to repeat the film deposition process and a patterning process at least twice, and these processes are usually repeated three to four times.
The switching element is formed according to the specific characteristics of a pixel. Because the area of the switching element is smaller than the area of the pixel, various thin films (a metallic layer, an insulating layer and a semiconductor layer for instance) which are deposited on almost the entire surface of the substrate are mostly eliminated in the patterning process. For instance, in the case of a transmissive liquid crystal display, approximately 90 percent of the metallic layer and semiconductor layer is eliminated in order to increase a pixel open area ratio.
The patterning process is mostly performed by a photolithographic process using photomasks, where it is necessary to align the photomasks with high precision in each process in order to suppress variations in element characteristics.
Furthermore, the substrates are becoming larger in size due to increasing size of display devices or for the sake of improving manufacturing efficiency, and so an apparatus for performing the film deposition process and patterning process is becoming larger in size.
As described above, the active matrix type FPD in the past has most of the materials for forming the switching element removed so that, as resources and energy are lost, costs thereof increase and a load to the environment also increases.
In addition, for the purposes of coping with the larger-size substrates, an expensive film deposition apparatus and a photolithographic apparatus for forming the switching element become larger in size. For that reason, manufacturing costs increase, and it becomes difficult to form the switching element having little variations in element characteristics over a large area.
As a method of solving these problems, the applicant of the present application proposed in Japanese Patent Application No. 2001-374559 an active matrix type display device which can be manufactured by an easier method than before by using a bidirectional two-terminal element produced in a process independent from a manufacturing process of the display device and a manufacturing method thereof. It should be noted that Japanese Patent Application No. 2001-374559 was not published prior to the filing date of the Japanese Priority Application for the present patent application.
In addition, U.S. Pat. No. 5,715,026 is relevant to the present invention and discloses a method for producing an active matrix type display device by using an individual non-linear active element produced beforehand.
In the configuration shown in FIG. 21, and similarly in the configuration disclosed in U.S. Pat. No. 5,715,026, a display device 900 disclosed in the above-identified Japanese patent application has a first electrode 91 and a picture electrode 14 electrically connected to the first electrode 91 via at least one bidirectional two-terminal element 12. A predetermined voltage is applied to a display medium layer (not shown) by a second electrode (not shown) arranged to be opposed to the picture electrode 14 via the display medium layer so as to perform display.
In the configuration shown in FIG. 21, however, there are cases in which the area of the region in which the first electrode 91 and the picture electrode 14 are overlapping is so large that a parasitic capacitance between the first electrode 91 and picture electrode 14 becomes significant so as to degrade display quality due to shadowing and other disadvantageous effects.